The present invention relates to an electrophotographic image forming system (method and apparatus) including a specific system) using non-single crystal silicon-based electrophotographic photosensitive member, a contact charging means and a spherical toner and including no cleaning step between the transfer and charging system, and more particularly to such an electrophotographic image forming system using a non-single crystalline silicon photosensitive member provided with a surface layer having a silicon content (percentage of silicon versus total of silicon and carbon).
A conventional electrophotographic image forming method generally includes a cycle of charging-exposure-development-transfer-cleaning of transfer residual toner-residual charge removal of the photosensitive member-charging (in a subsequent cycle). According to this, transfer residual toner remaining on the photosensitive member (image-bearing member) after the transfer step is removed from the photosensitive member surface and recovered as waste toner by a cleaner (cleaning device). It is desired that such waste toner does not occur from a viewpoint of environmental protection. Accordingly, there has been proposed an electrophotographic apparatus of a toner-recycle type having removed such a cleaner and including a developing device for simultaneous developing and cleaning for removing transfer residual toner on the photosensitive member and recover it in the developing device for reuse. Such an electrophotographic apparatus is disclosed in, e.g., Japanese Laid-Open Patent Application (JP-A) 10-307455.
Simultaneous developing and cleaning is an operation of removing the transfer residual toner on the photosensitive member in a developing step of a subsequent cycle, i.e., after charging and exposing the photosensitive member to form a latent image, under the action of a fog-removing bias voltage difference Vback (i.e., a potential difference between a DC voltage applied to the developing device and the surface potential on the photosensitive member). According to this method, the transfer residual toner is recovered in the developing device and re-used in a subsequent image forming cycle or later, thus removing the waste toner and reducing the troublesome maintenance operation. The cleanerless system is advantageous for space economization, thus providing an electrophotographic apparatus remarkably reduced in size.
Hereinbelow, such a toner recycle process including a simultaneous developing and cleaning step will be briefly described with reference to FIG. 3.
(1) A photosensitive member 201 is supplied with a voltage by a contact charging member 202 to be uniformly charged to, e.g., a negative polarity in this case.
(2) The uniformly negatively charged photosensitive member 201 is exposed to light 203 carrying image data (e.g., laser light) for the reversal development scheme, to form a latent image thereon.
(3) A charged toner 205 comprising colored powder is supplied from a developing device 204 in a form corresponding to the latent image onto the photosensitive member 201 surface to form a visible toner image thereon. The toner in this case is a negatively charged one.
(4) By applying a voltage from a transfer roller 206 or by the action of a member exerting an electrostatic attraction force, the toner image is transferred onto a recording material (or transfer (-receiving) material) 207 to be fixed thereon by a fixing device 209. In this instance, a portion of the toner is caused to remain as a transfer residual toner without being transferred, and a portion of the residual toner is liable to be charged in a polarity opposite to the polarity to which the toner is normally charged by a voltage of an opposite polarity (e.g., positive) applied to the transfer roller 206. The portion of the transfer residual toner charged to the opposite polarity may be referred to as an inverted toner 208.
(5) The transfer residual toner remaining on the photosensitive member surface including the above-mentioned inverted toner 208 is re-charged to the normal toner charge polarity by rubbing between the contact charging member 202 and the photosensitive member 202 in the step (1) of a subsequent cycle and then discharged onto the photosensitive member. The charging of the invented toner to the normal polarity may be referred to a normalization of toner polarity.
(6) The residual toner re-charged to the normal polarity is recovered to the developing device 204 together with excessive toner supplied at the time of development under the action of a developing bias voltage.
By repeating the above-mentioned cycle, a system yielding no waste toner is realized. The above explanation is a summary and not an exhaustive description of such a cleanerless system.
Incidentally, JP-A 10-307455 discloses a toner recycle process using a non-single crystalline silicon photosensitive member. Further, JP-A 2000-98846 contains a disclosure regarding contact charging of a non-single crystal material having a surface layer comprising non-single crystalline carbon.
Further, Japanese Patent (JP-B) 2811312, etc., discloses a composition represented by a formula of Si1xe2x88x92xCx with a value x of 0.5 less than x less than 0.9, and JP-A 10-20663 discloses a composition represented by a formula of Si1xe2x88x92xCx with a value x of 0.95xe2x89xa6x less than 1.
On the other hand, JP-A 10-213946 discloses a cleanerless process wherein a voltage lower than a saturation potential of a photosensitive member is applied to a charging means to improve the toner discharge. JP-A 10-2774884 contains a disclosure to the effect that it is desirable to normalize the toner charge in a toner recycle process. Further, JP-A 9-325578 discloses an auxiliary charging mean for charging the residual toner to an opposite polarity before the ordinary charging step.
A non-single crystalline silicon photo-sensitive member has a superior potential stability not comparable to any other photosensitive members and has been frequently used in high-speed copying machines and high-speed printers. Particularly, a photosensitive member comprising a photoconductor layer comprising a silicon-based non-single crystal material and a surface layer comprising a carbon-based non-single crystal material successively formed on an electroconductive support has excellent properties of sticking prevention, long life and environmental stability because of the characteristics of the non-single crystalline carbon film, such as lubricity, high hardness and oxidation resistance.
The potential stability is also desired in small-size machines and popular machines, but these machines are disposed after printing on at most one million sheets. This is not matched with a durability on several million sheets of a non-single crystalline silicon photosensitive member, thus posing an obstacle to wide popularization of the non-single crystalline silicone photosensitive member.
Further, when used in the above-mentioned cleanerless system, some among a variety of non-single crystal material silicon photosensitive members is liable to convert the inverted to a polarity distribution not readily recovered by the fog-prevention voltage, more specifically, to an average polarity opposite to the normal charge polarity, thus posing a difficulty for realizing a cleanerless system capable of fully enjoying the advantages of a non-single crystalline silicon photosensitive member.
Particularly, compared with a surface layer comprising an organic material such as a resin, a surface layer comprising a carbon-based non-single crystal material is liable to make difficult the normalization of inverted toner, thus causing image defects, such as image fog and lower image density due to deterioration of the developer.
It is particularly important that the toner and the photosensitive member have an identical charge polarity so as to ensure an IAE (image area exposure) scheme wherein an image forming part or region on the photosensitive member is exposed to have a part of lower potential which is to be developed with the toner. The toner mixed in the charger is charged in the above-described manner and discharged in a normally charged polarity onto the photosensitive member by resolution against the charging member, followed by successful recovery under the developing bias voltage. On the other hand in a BAE (back area exposure) scheme wherein a region other than an exposed (i.e., charge-attenuated) part is developed with a toner charged to an opposite polarity, there are liable to be encountered difficulties that the toner fails to be discharged because of an insufficient charge or fails to be recovered even if it is discharged with a sufficient charge.
In view of the above-mentioned problems of the prior art, an object of the present invention is to provide an electrophotographic image forming system (method and apparatus) capable of well-operating a cleanerless toner recycle process while using a non-single crystalline silicon photosensitive member in a form well adapted to small-size machines and popular machines.
Another object of the present invention is to provide an electrophotographic image forming system capable of remarkably reducing the amount of waste material over an entire life of the system and stably providing clear images free from image flow or image irregularity for a long period in various environments.
Another object of the present invention is to provide an electrophotographic image forming system having a long life of charging member and capable of stably providing high-quality images with a minimum maintenance cost.
Another object of the present invention is to provide an electrophotographic image forming system wherein a non-single crystalline silicon photosensitive member is uniformly charged with no irregularity to provide clear images which are uniform, have a high contrast and a high resolution and are accompanied with little fog.
According to the present invention, there is provided an electrophotographic image forming method, comprising cyclic steps including:
a charging step of charging a rotating image-bearing member to charge a surface thereof,
a latent image forming steps of forming an electrostatic latent image on the charged surface of the image-bearing member,
a developing step of developing the electrostatic latent image with a magnetic toner to form a toner image thereon, and
a transfer step of transferring the toner image onto a recording material,
wherein the image-bearing member comprises an electroconductive support, and a photoconductor layer and a surface layer formed on the support; said photoconductor layer comprising a silicon-based non-single crystal material containing at least one of hydrogen and halogen, said surface layer comprising a carbon-based non-single crystal material containing at least one of hydrogen and halogen and also containing silicon in a proportion of 0.2 to 20 atm. % as calculated by Si/(Si+C),
said magnetic toner comprises toner particles comprising at least a binder resin and a magnetic material, and inorganic fine powder, has an average circularity of at least 0.950 and has a saturation magnetization of 10 to 50 Am2/kg as measured at 79.6 kA/m,
in the charging step, the image-bearing member is charged to a negative polarity by a contact charging means including charging particles comprising principally electroconductive particles having particle sizes of 0.1-10 xcexcm, and a charging particle carrying member having an electroconductive and elastic surface and carrying the charging particles on the surface so as to contact the image-bearing member via the charging particles,
in the latent image forming step, an image forming part of the surface of the image-bearing member is exposed to light to provide an attenuated potential thereat, thereby forming the electrostatic latent image, and
no cleaning step is included between the transfer step and the charging step.
According to another aspect of the present invention, there is provided an electrophotographic image forming apparatus, comprising:
a rotating image-bearing member,
a charging means for charging the rotating image-bearing member to charge a surface thereof,
a latent image-forming means for forming an electrostatic latent image on the charged surface of the image-bearing member,
a developing means for developing the electrostatic latent image with a magnetic toner to form a toner image thereon, and
a transfer means for transferring the toner image onto a recording material,
wherein the image-bearing member comprises an electroconductive support, and a photoconductor layer and a surface layer formed on the support; said photoconductor layer comprising a silicon-based non-single crystal material containing at least one of hydrogen and halogen, said surface layer comprising a carbon-based non-single crystal material containing at least one of hydrogen and halogen and also containing silicon in a proportion of 0.2 to 20 atm. % as calculated by Si/(Si+C),
said magnetic toner comprises toner particles comprising at least a binder resin and a magnetic material, and inorganic fine powder, has an average circularity of at least 0.950 and has a saturation magnetization of 10 to 50 Am2/kg as measured at 79.6 kA/m,
the charging means includes charging particles comprising principally electroconductive particles having particle sizes of 0.1-10 xcexcm, and a charging particle-carrying member carrying the charging particles so as to contact the image-bearing member via the charging particles, thereby charging the image-bearing member to a negative polarity,
the latent image-forming means includes an exposure means for exposing an image forming part of the image-bearing member to provide an attenuated potential thereat, and
no cleaning means is present between the transfer means and the charging means along the surface of the image-bearing member.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.